+/* Structure for defining custom output streams. You will need to provide
+ * a callback function to write the bytes to your storage, which can be
+ * for example a file or a network socket.
+ *
+ * The callback must conform to these rules:
+ *
+ * 1) Return false on IO errors. This will cause encoding to abort.
+ * 2) You can use state to store your own data (e.g. buffer pointer).
+ * 3) pb_write will update bytes_written after your callback runs.
+ * 4) Substreams will modify max_size and bytes_written. Don't use them
+ * to calculate any pointers.
+ */
+struct _pb_ostream_t
+{
+#ifdef PB_BUFFER_ONLY
+ /* Callback pointer is not used in buffer-only configuration.
+ * Having an int pointer here allows binary compatibility but
+ * gives an error if someone tries to assign callback function.
+ * Also, NULL pointer marks a 'sizing stream' that does not
+ * write anything.
+ */
+ int *callback;
+#else
+ bool (*callback)(pb_ostream_t *stream, const uint8_t *buf, size_t count);
+#endif
+ void *state; /* Free field for use by callback implementation. */
+ size_t max_size; /* Limit number of output bytes written (or use SIZE_MAX). */
+ size_t bytes_written; /* Number of bytes written so far. */
+
+#ifndef PB_NO_ERRMSG
+ const char *errmsg;
+#endif
+};
+